This invention relates to coating of glass bulbs or tubes. More particularly it relates to an apparatus and method for coating the interior surface of glass bulbs or tubes.
Glass bulbs or tubes, which are used as the envelope of fluorescent lamps, are generally coated with one or more layers of phosphor. The phosphor coating transforms some of the ultraviolet energy generated by an electrical discharge within the bulb into visible light.
A color rendering index, which is a measure of the degree to which the perceived colors of objects illuminated by light sources conform to those of the same objects illuminated by a reference source, typically sunlight, under specific conditions, is generally used to determine light quality. It has been found that color rendering of a fluorescent lamp can be generally improved by applying multiple layers of phosphors, each of which may have a different composition. Further, the degradation of phosphors over time may be decreased by applying a protective coating of a non-depreciating material such as a protective phosphor. In applying these multiple coatings of phosphors, it is desirable that each layer of phosphor be uniform throughout the length of the bulb e.g. same thickness, same phosphor content per unit area, etc., in order that the light produced along the length of the bulb is uniform.
In a current commercial coating process, a predetermined amount of a phosphor slurry is introduced into an open end of a vertical glass tube to contact the sides thereof. The slurry is permitted to flow down the sides of the tube forming a film of phosphor thereon. Any excess slurry flows out the open bottom of the tube and is collected and recycled, which may lead to impurities being added and to expensive attempts to maintain a clean environment. In this process, control over the slurry and tube parameters is crucial in order to obtain uniform coating. This control is generally provided by controlling the viscosity, total solids (phosphors) and temperature of the slurry and temperature and surface wetness of the tube. Even with such controls, it is difficult to ensure that the thickness and phosphor content per unit area of the coating from one end of the tube to the other are uniform.
When a multiple phosphor coating is to be used, as for instance to increase the color rendition of the lamp or to slow the degradation of a previously coated or base phosphor, it is preferable that the additional or secondary phosphor be applied in a relatively thin uniform coating. The secondary phosphors, which may improve color rendition or inhibit the degradation of a previously deposited, or base, phosphor, are traditionally much more expensive that the base phosphors. Thus, in order to avoid excess and waste it is desired to deposit only as much of the secondary phosphor as will actually be effective for its intended purpose. Such stringent control over the coating process is impractical with the flow coating method presently employed.